Liquid crystals (LCs) are materials that flow like liquid with crystalline solid-like ordered molecules that align and orient along a particular direction in the presence (or absence) of an electric field. These materials are widely used to manipulate the polarization and transmission of light, including in liquid crystal displays (LCDs). In an LCD, an LC layer is usually formed by aligning the LC material with respect to a pair of substrates and sandwiching the substrates between a pair of crossed polarizers. Applying an electric field to the LC layer causes the LC to align or twist, thereby allowing or blocking the incident light.
Typically, the LC material is aligned to the substrate with an alignment layer. The alignment layer is typically applied through a standard spin-coating method with a layer thickness on the order of several hundred nanometers. This layer orients the LC molecules, which often have an oblong shape, along a surface of the substrate, which is typically transparent glass or plastic. This type of alignment causes most or all of the LC material to form a “single crystal” that can be re-oriented using an electric field. Absent this alignment layer, the liquid crystals would behave as a “polycrystalline” material; that is, the LC layer would form smaller LC domains, each containing molecules aligning in an orientation different from those of other LC domains. Light passing through a polycrystalline LC layer undergoes non-uniform scattering and random variation in light transmission, producing diffused, low-intensity lighting.
One of the most common ways to provide liquid crystal alignment is by first coating the surface with a thin film of polymer, such as polyimide, and then rubbing the surface with a cloth. The cloth aligns the polymer molecules on the surface in the rubbing direction; the liquid crystal in contact with the surface aligns to the polymer molecules. This approach has been quite effective, and thus widely used in the LCD industry. Although this rubbing alignment technique is generally applicable to display technologies that work with large, flat display platforms and substrates, it may not be applicable to particular LC applications that utilize non-planar, non-standard, and/or smaller cavities to hold LC. In addition, the rubbing an alignment layer with a cloth tends to generate particles, making it suitable for certain applications.
Other techniques for aligning LC materials include a photoalignment technique, which utilizes polarized light to form an alignment layer for LC materials. Photoalignment case be used in a variety of non-standard geometries. For instance, photoalignment has been utilized in the creation of a tunable microresonator in which the alignment layer is applied through a standard spin-coating method. Great success has also been shown in the use of photoalignment for tunable photonic crystal fibers (PCFs). In this case, the application of the photoalignment layer via spin-coating is not possible; instead, the fiber is filled with the photoalignment solution through capillary action into the fibers, then excess solution is removed through a pressure gradient.
There are a number of different photoalignment techniques which can be categorized by the way in which the polarized irradiation causes surface anisotropy: the polarized light can result in polymerization with cross-linking along one direction (photo-polymerization), it can result in degradation of molecules aligned along one direction (photo-degradation), it can result in a conformational change of molecules along one direction (photo-isomerization), or it can excite molecules preferentially along one direction (photo-reorientation). The last two of these are most commonly accomplished using azo dyes which often absorb well in the ultraviolet (UV) or visible range. While photo-isomerization is frequently criticized as having poor lifetime due to the gradual relaxation of molecules from the cis- to the trans-state, photo-reorientation, depending on the relaxed molecular conformation, is a much more attractive choice because it can be excited preferentially along the polarization. In addition to the lower irradiation energies compared to both photo-polymerization and photo-degradation, photo-reorientation of azo dyes results in an alignment layer with an order parameter which can be even higher than the liquid crystalline order parameter.
In photo-reorientation, a dichroic dye, most often one containing azo groups, is irradiated with polarized light of an appropriate wavelength (i.e., one which is well absorbed by the dye). The probability that a given dye molecule will absorb this incident irradiation is proportional to cos2θ where θ is the angle between the incident polarization axis and the long axis of the dye molecule. Over time, this absorption increases the population of dye molecules aligned perpendicular to the incident polarization, where the probability of absorption is at or near zero. After a sufficient exposure dose, the order parameter, which is determined by the absorption spectra of the dye both parallel and perpendicular to the polarization axis of the irradiating light, can exceed even that of the liquid crystals it is being used to align.
Anchoring energies of these layers have also been measured to be on the same order of magnitude as the anchoring achieved through rubbed polyimide alignment. This is particularly important in photonic devices where light scattering from director fluctuations can degrade device performance. Anchoring energies on the range of that observed from polyimide and also for azo-dye alignment layers suppress these fluctuations to an acceptable level in some devices. It should be noted that director fluctuations are not a large concern for display devices.
Unfortunately, conventional photo-aligned layers tend to degrade when exposed to light or heat, making them unsuitable for many applications, including displays and thermal sensing. Of particular importance for photonic applications is stability under exposure to light of random polarization states. Also, in the case of photonic devices, the light intensity which the device is subjected to can be quite high, enhancing the probability of device failure if the stability is low. It should be noted that for many applications of azodye alignment layers, the “rewriteability” of these materials is emphasized as a positive attribute. However, in the case of photonic devices where the azodyes are desired for their high anchoring energy, rewriteability is problematic.